The final GOP debate of 2015 is tonight! Here's what you need to know

U.S. Republican presidential candidate Donald Trump waves as he leaves the stage after speaking at the meeting of the New England Police Benevolent Association in Portsmouth
Thomson Reuters
With the first voting contests less than 50 days away, Republican candidates are set to take the stage for the fifth and final time of 2015.

Much has changed since that September affair. Two candidates — Wisconsin Gov. Scott Walker (R) and former Arkansas Gov. Mike Huckabee (R) — are gone from the main stage. (Walker dropped out of the race, while Huckabee's polling numbers did not allow him to qualify.)

The nine candidates will take center stage at 8:30 p.m. ET Tuesday night. Four other, lower-polling candidates will square off in a separate, "undercard" debate at 6 p.m. ET on the network.

The polling

According to Real Clear Politics' average of seven recent national polls, here's a look at where the candidates stand entering Tuesday night. From the top tier:

Donald Trump, real-estate magnate: 31.4% average as of Tuesday (up from a 24.8% average before the November Fox Business debate)

Ted Cruz, US senator from Texas: 16.3% (up from 9.6%)

Marco Rubio, US senator from Florida: 13.3% (up from 11.8%)

Ben Carson, retired neurosurgeon: 12.6% (down from 24.4%)

Jeb Bush, former Florida governor: 4% (down from 6%)

Carly Fiorina, former Hewlett Packard CEO: 2.6% (down from 3%)

Chris Christie, New Jersey governor: 2.6% (up from 2.2%)

John Kasich, Ohio governor: 2.3% (down from 3%)

Rand Paul, US senator from Kentucky: 2.1% (down from 3%)

CNN

And the bottom tier:

Mike Huckabee, former Arkansas governor: 2% (down from 2.4%)

Rick Santorum, former US senator from Pennsylvania: 0.2%

Lindsey Graham, US senator from South Carolina: 0.2%

George Pataki, former New York governor: 0.2%

Other notable changes since last time:

Chris Christie is back on the main stage after being relegated to the "undercard" debate in November, having met the average polling threshold in New Hampshire surveys.

Bobby Jindal, who was part of the past four "undercard" debates, dropped out of the race last month.

Jeb Bush during an interview with Business Insider.
Graham Flanagan/Business Insider

The themes

Are Trump and Cruz on a collision course? As Cruz has surged to challenge Trump's front-running status in Iowa, the real-estate mogul has sharpened his attacks on the senator. Trump's first attack came after Cruz, in leaked audio from a private fundraiser, questioned Trump's "judgment" as a potential commander-in-chief. Trump spent most of the weekend positing relatively subtle attacks, but earned some backlash from the conservative talk-show faithful on Monday. Will he continue escalating on the main stage? "If there is an underlying theme for tonight's debate, we suspect it will be a Ted Cruz pile-on led by Donald Trump and Marco Rubio," said Chris Krueger, an analyst at Guggenheim Securities.

What of Rubio's 'wins?' Rubio has been the pundit-proclaimed "winner" of at least two or three of the four debates thus far. He has seen a slow rise in his poll numbers ever since August's first debate, but has so far been see the kind of surge that has propelled Cruz recently. Cruz, meanwhile, has outflanked him in the key early state of Iowa, while Christie is nipping at his heels for second place in New Hampshire. Tuesday is again a big night for Rubio.

Speaking of Christie... Can he turn the mini-boomlet in New Hampshire into something nationally? He benefited in November from being relegated to the "undercard" debate, where he overmatched his competitors. He has a chance to continue the momentum Tuesday night.

U.S. Republican presidential candidate Chris Christie speaks at the Growth and Opportunity Party at the Iowa State Fairgrounds in Des Moines, Iowa
Thomson Reuters

Can Ben Carson turn around a campaign in free fall? Carson has arguably had the worst month of any GOP candidate, as questions about his background foreign-policy chops dominated the news cycle for weeks. (The foreign-policy questions began after an odd claim during the last debate.) His poll average has crumbled by half over the past month. Is there another moment somewhere in Tuesday's debate for him?